General philosophy
If you can get the job done with a simple method, it's far better than using a more complicated method just because it maybe looks sexy or whatever.
So keep it simple.
Simple is often fast. Simple is easy. Simple is debuggable. Simple is stable. All things have exceptions, but often simpler is just better.
Simplicity is about subtracting the obvious and adding the meaningful.
Psychology
From a psychological point of view, the KISS principle is an application of the primacy effect 1)
Occam's razor
wiki/Occam's_razor principle states that among competing hypotheses, the one with the fewest assumptions should be selected.
“other things being equal, simpler explanations are generally better than more complex ones”.